1. Field of the Invention
This invention relates to a method for producing a sintered porous bearing and the sintered porous bearing for use in, for example, spindle motors rotating at high speeds and a production method therefor, and more particularly to the method for producing a sintered porous bearing and the sintered porous bearing which eliminates loss of energy due to abrasion by high speed rotation, eliminates increase of abrasion due to vibration and the like and enables to be used in a long-term.
2. Description of the Related Art
Conventionally, a sintered porous bearing (hereinafter referred to as bearing) in which a middle portion along an axis of an internal circumferential portion which contacts a shaft has a larger diameter than that of both ends and is expanded toward the inside of the bearing substance has been known. This type of the bearing is subjected to a small rotation resistance against the shaft because the expanded portion in the middle portion is not in contact with the shaft and therefore suitable for supporting the shaft rotating at high speeds. For example Japanese Patent Publication No. 60-54525 has disclosed a art that by using die having a gap for part of an external circumferential portion of a sintered object, the sintered material having straight cylindrical faces of its internal and external circumferential portions is recompressed so as to expand its middle portion along its axis of the sintered material toward outside in the radius direction thereby forming a middle expanded portion in the bearing substance. The similar arts have been disclosed in Japanese Patent Publication No. 6-4883, Japanese Patent Application Laid-Open No. 62-151502. Further, Japanese Patent Application Laid-Open No. 2-8302, Japanese Patent Application Laid-Open No. 3-240901 and Japanese Patent Application Laid-Open No. 1-242821 have disclosed an art that the internal circumferential portion of a bearing is formed in cylindrical form having steps including a large diameter portion and a small diameter portion and then an end portion of the large diameter portion is drawn so as to reduce the diameter thereof thereby forming a middle expanded portion.
According to the above described conventional arts, the external circumferential portions of the bearings in any cases are not straight because of stepped formation, then mounting those bearings onto a housing is difficult. That is, because the external circumferential portion of the large diameter portion of the bearing is not restrained at the time of recompression, not only a disparity occurs in dimensional accuracy but also the bearing is supported unstably so that a large load cannot be supported when it is mounted on the housing because it is supported only by either the large diameter portion or the small diameter portion. Further, with respect to art of reducing the diameter by drawing the end of the large-diameter portion, a difference occurs in internal porosity because a degree of processing differs between both ends of the bearings.
Additionally, Japanese Patent Publication No. 63-43611 and Japanese Patent Publication No. 4-45962 have disclosed such art that first a concave portion is formed such that it extends around an entire circumferential portion of a middle portion in axial direction of an external circumferential portion of a bearing by mechanical processing and then by recompressing the bearing in the axial direction, the concave portion is made to expand toward the outside so as to form an expanded portion on an internal circumferential portion thereof. However, because the sintered porous bearing is superior in some respect to other bearings because it can be produced at a relatively low cost, cutting processing is actually impossible in viewpoints of production cost.
Further, Japanese Patent Application Laid-Open No. 2-107705 has disclosed an art that at the time of sizing, a large diameter portion formed on a core is positioned in a middle portion in the axial direction of a bearing, the bearing is compressed along the axial direction, the bearing is drawn out of a die together with the core and the core is pulled out of the bearing. According to this art, although an external circumferential portion of the bearing can be formed in straight form, because the bearing is subjected to elastic deformation when the core is pulled out, a dimension of the large diameter portion formed on an internal circumferential portion of the bearing can not be set large and further setting of the density of the bearing is also restrained.
According to Japanese Utility Model Application Laid-Open No. 61-124702, a concave portion is formed around an entire surface of a middle portion in the axial direction of an external circumferential portion of a bearing, this bearing is pressed into an inside of a housing case and the diameter thereof is reduced corresponding to a distance of the pressing so as to form a middle expansion in an internal circumferential portion thereof. However, this art has such disadvantages that a high precision is required in the internal dimension of the housing case and material thereof is limited.
According to the above arts, contact area is reduced by forming the middle expanded portion on the internal circumferential portion of the bearing. To attain the same effect, for example Japanese Patent Publication No. 42-15447 has disclosed an art that a plurality of longitudinal grooves extending in the axial direction are formed on an internal circumferential portion thereof. According to Japanese Patent Publication No. 44-15683, a plurality of longitudinal grooves are formed on an internal circumferential portion of the bearing and after that, by correcting the entire internal circumferential portion to a real circular shape as viewed in section, a density of a portion having grooves is adjusted so as to be low and a density of a portion having no grooves is adjusted so as to be high. Further, Japanese Utility Model Publication No. 47-36739 has disclosed a bearing in which the density of a portion containing a longitudinal groove is made low and at the same time the density of a portion between the longitudinal grooves is made high. Further, Japanese Patent Publication No. 47-50899 has disclosed a bearing in which a sectional shape of an internal circumferential portion thereof is constituted of plural circles having a larger curvature than a radius of a shaft. In this bearing, clearance between the bearing and the shaft varies along its circumferential portion and the shaft is supported by hydraulic pressure produced when lubricant is sucked from a portion having a large clearance to a portion having a small clearance.
A number of arts similar to the aforementioned bearings have been proposed and further various groove types such as angled grooves and spiraled grooves have been proposed. A common purpose of these proposals is to reduce a rotational resistance of a shaft or concretely speaking, a resistance (resistance by fluid lubrication) between an internal circumferential portion of a bearing and an external circumferential portion of a shaft which is a particular problem in high speed rotations. However, although such an object is attained by these proposals, they have such disadvantages that hydraulic pressure drops because lubricant leaks outside from both ends of the bearing through its grooves and lubricant is seriously consumed. These result in drop of bearing performance. Although various kinds of bearings for high speed rotation have been proposed since before, any one of them has its own disadvantages.
The present invention has been achieved with such points in mind.
It therefore is an object of the present invention to provide a method for producing a sintered porous bearing in which production of the bearing is very easy.
It is another object of the present invention to provide a method for producing a sintered porous bearing in which a external circumferential portion of the bearing can be formed in a straight form.
It is still another object of the present invention to provide a sintered porous bearing on which wear resistance can be reduced because density in a slide portion thereof is high.
It is further object of the present invention to provide a sintered porous bearing in which an external circumferential portion thereof is formed in straight form and which has a high performance.
To achieve the objects, according to a first aspect of the present invention, there is provided a method for producing a sintered porous bearing comprising the steps of: placing a cylindrical sintered material in a cavity composed of a die, a core having a smaller diameter than an internal diameter of the sintered material and disposed in the center of the die, and punches provided within the die; compressing the sintered material by the punches in an axial direction so that internal circumferential portions of both ends of the sintered material protrude inward in a radius direction in a gap between the internal circumferential portion of the sintered material and an external circumferential portion of the core; and further compressing the sintered material by the punches in the axial direction till the internal circumferential portions of both ends of the sintered material come into contact with the external circumferential portion of the core to form slide portions sliding on a shaft to be inserted into the sintered porous bearing.
According to a second aspect of the present invention, as it depends from the first aspect, an end face of the sintered material is provided with a protruding edge for protruding in the axial direction for coming into contact with an end face of the punch.
According to a third aspect of the present invention, as it depends from the first or the second aspect, an end face of the punch is provided with a protruding edge for protruding in the axial direction for coming into contact with an end face of the sintered material at the vicinity of the of the internal circumferential portion.
According to a fourth aspect of the present invention, as it depends from one of the aspect among the first aspect to the third aspect, an end face of the sintered material is provided with a protruding edge for protruding in the axial direction for coming into contact with an end face of the punch; and the face of the punch is provided with a protruding edge for protruding in the axial direction for coming into contact with the face of the sintered material at the vicinity of the of the internal circumferential portion.
According to a fifth aspect of the present invention, as it depends from one of the aspect among the first aspect to the fourth aspect, the sintered material is formed with a concave portion extending in the axial direction at the internal circumferential portion thereof, the concave portion extends from an end to the other end of the internal circumferential portion thereof.
According to a sixth aspect of the present invention, as it depends from one of the aspect among the first aspect to the fifth aspect, in the step further compressing the sintered material for forming the slide portion, the internal circumferential portions of both ends of the sintered material protrudes in the radius direction so as to come into contact with the core till the concave portion formed on the internal circumferential portion of the sintered material is flattened to be vanished.
According to the above production method, because there is provided a gap between the internal circumferential portion of the sintered material and the external circumferential portion of the core, plastic flow is produced in the vicinity of both ends of the sintered material which is pressed directly by the punches so that the internal circumferential portion is protruded in a direction of the core which is not restrained. As a result, the internal circumferential portions at both ends of the sintered material are contracted so that a middle expanded portion in which an internal diameter of a middle portion is larger is produced. Because according to the present invention, the sintered material is compressed in the axial direction so that the middle expanded portion is formed as described above, the external circumferential portion of the sintered material can be formed in straight form. However, the present invention does not exclude modifications in which the external circumferential portion is provided with steps, tapered or formed in other configurations.
In such an art in which a middle portion in the axial direction of the sintered material is expanded outward in the radius direction as conventional arts, formation of grooves in an internal circumferential portion of die (Japanese Patent Publication No. 60-54525) or machining of circular grooves in the sintered material (Japanese Patent Publication No. 63-43611) is necessary. The present invention requires no such treatment and therefore production of the bearing is very easy. Further, although conventional arts are not capable of producing products having a short length in the axial direction, the above production method has no restriction in terms of product length. Further, because end portions of the sintered material are not drawn, setting of its entire density is relatively free and a difference in step of the middle expanded portion can be set so as to be relatively large.
Here, compression by punches is desired to be carried out until the internal circumferential portion at both ends of the sintered material is pressed up to the external circumferential portion of the core. Consequently, the configuration of both end portions in contact with a shaft becomes the same as the external circumferential portion of the core (e.g., circular in section) so that a desired configuration and dimension can be formed. Further because the internal circumferential portions at both ends of the sintered material are pressed up to the external circumferential portion of the core, the density of a portion facing the internal circumferential portion of the slide portion is intensified thereby realizing production of an ideal bearing.
According to a seventh aspect of the present invention, there is provided a sintered porous bearing, comprising: a hole portion engaging with a shaft to be inserted into the sintered porous bearing; and slide portions provided on an internal circumferential portion of the hole portion at both ends thereof in an axial direction, the slide portions coming into contact with the shaft, wherein an internal diameter of the slide portion is smaller than a diameter of an internal circumferential portion in a middle portion of the hole portion; and a density of the internal circumferential portion of the slide portion is higher than the density of an external circumferential portion of the slide portion.
Here, in the hole portion, the middle portion in the axial direction can be expanded with a larger diameter than both ends, however it is permissible to so construct that part of the middle portion in the axial direction is in contact with the shaft. Concretely speaking, it is possible to provide the middle portion in the axial direction of the cylindrical hole portion (e.g., cylinder) with one or two or more concave portions extending along the axial direction. This concave portion may be communicated with end faces of the sintered material or may be formed so that it does not reach the end faces. In this case, a bottom of the concave portion is a portion most outside of the internal circumferential portion or a portion forming the most outside diameter of the hole portion.
In this bearing, its wear resistance is excellent because the density in the slide portion is high. By providing the other portions with a low density (that is, increasing the porosity), it is possible to contain a sufficient amount of lubricant. Accordingly, the service life of the bearing can be extended long without providing with any special oil supplying mechanism for supplying lubricant to the external circumferential portion of the bearing.
To obtain a shape of the bearing in which the middle portion is expanded, providing at least one of end faces on the internal circumferential side of the sintered material and end faces to be in contact with the sintered material, of the punches, with a protruding edge which is protruded in the axial direction is effective. According to this production method, plastic flow at both ends of the sintered material is large so that the difference of step of the middle expanded portion can be further increased. Meantime, it is permissible to provide the protruding edge around an entire circle of the end face continuously or intermittently. Further, it is permissible to provide it in any position of the middle position, on the internal circumferential side and the external circumferential side in the radius direction.
According to a eighth aspect of the present invention, there is provided a sintered porous bearing, comprising: a hole portion engaging with a shaft to be inserted into the sintered porous bearing; and slide portions provided on an internal circumferential portion of the hole portion at both ends thereof in an axial direction, the slide portions coming into contact with the shaft, wherein a diameter of an internal circumferential portion in a middle portion of the hole portion is larger than an internal diameter of the slide portion; and the hole portion is provided with a concave portion extending in the axial direction at the internal circumferential portion thereof.
Lubricant is stored in a gap between the middle portion in the axial direction of this bearing and a shaft such that lubricant is supplied to the slide portions. Although this point is the same as conventional examples described above, this bearing is characterized in having the concave portions in the middle expanded portion.
Because the bearing according to the second aspect of the present invention is provided with concave portions in the middle expanded portion, even if the difference of the step of the middle expanded portion is small, lubricant can be stored in the concave portions so that storage amount of lubricant is sufficient. Further, because a distance to the shaft is increased in the concave portion, flow resistance of lubricant can be reduced. Further, an effect of dynamic pressure is produced because lubricant in the concave portions is sucked to a portion having a small clearance, so that deflection of the shaft can be further effectively reduced. Thus in the bearing according to the second aspect, there is no any inconvenience although the difference of the step in the middle expanded portion is small.
Generally, the slide portions are subjected to machining such as reaming before actual use. Needless to say, it is easy to determine concentricity for two slide portions because the area to be machined is small.
As the concave portions, one or two or more grooves extending along the axis are preferable and they may be either parallel to the axis or spiraled. Further, the concave portions may be contained within the middle expanded portion, reach the slide portions or communicate with end faces of the bearing.
In case the concave portions are disposed so that they are contained within the middle expanded portion, an area of the slide portion is secured so that a capacity of supporting oil film (bearing rigidity) is intensified. Because leakage of lubricant to outside is not produced, wear of the shaft and bearing is small and thus any outside oil supplying mechanism is not required. Further because lubricant pressurized in the concave portions is forced to exude into the slide portions, supply of lubricant is carried out sufficiently. This operation and effect can be obtained even in such a structure in which the concave portions extend up to the inside of the slide portions.
Meanwhile, the concave portions described above may be applied to the first bearing of the present invention and therefrom the same operation and effect can be obtained.
According to a ninth aspect of the present invention, there is provided a sintered porous bearing, comprising: a hole portion engaging with a shaft to be inserted into the sintered porous bearing; and a concave portion formed, in a middle portion in an axial direction of the hole portion, the concave portion extending from an end of the hole portion to the other end thereof, the concave portion not reaching the each end of the hole portion.
In this bearing as well, lubricant is stored in the concave portions like in the second bearing and lubricant pressurized within the concave portions is forced to be exuded to the slide portions. Thus the slide portions are supplied with a sufficient amount of lubricant. Further, because leakage of lubricant to outside is not produced, consumption of lubricant is small and any outside oil supplying mechanism is not required. Further, because deflection of the shaft can be effectively reduced due to an effect of dynamic pressure and some area is secured in the slide portions, the capacity of supporting oil film is intensified. As the concave portions, one or two or more grooves extending along the axis are preferable and they may be either parallel to the axis or spiraled.
Although a production method for the bearing having the aforementioned concave portions may be selected arbitrarily, some device is needed to form the concave portions such that they do not reach the end faces of the sintered material. Thus in the present invention, such processing is carried out in following manner.
That is, according to the production method for the bearings of the present invention, concave portions extending from an end to the other end are formed on the internal circumferential portion of the sintered material and then the aforementioned production steps of the present invention are carried out so as to make the internal circumferential portions at both ends of the sintered material protrude inward in the radius direction and vanish the concave portions on the slide portions.
According to this production method, there is no problem if the concave portions formed on the internal circumferential portion of the sintered material communicate with both end faces, therefore production thereof is very simple. As for a forming method for the concave portions, it is desirable to provide protrusions (protruding edges) on a core for use in formation of green compact or sizing, however they may be formed by machining the sintered material. Further it is possible to process such that the concave portions on the internal circumferential portion at both ends of the sintered material are not vanished.